G protein-coupled receptors (GPCRs) class is the largest family of cell-surface receptors which plays a crucial role in intracellular signal transduction. Adenosine receptors are part of the GPCR class, which belongs to the Class A or rhodopsin-like subfamily of GPCRs. Adenosine, a purine nucleoside, produces numerous physiological actions via cell surface adenosine receptors. These receptors are widely distributed throughout the body and are divided into four subclasses, A1, A2A, A2B and A3 receptors, the latter being the most recently identified receptor.
The A3 adenosine receptor (A3AR) is involved in a variety of physiological processes. The receptor is highly expressed in various tumor cell types while expression in adjacent normal tissues is relatively low. Activation of the receptor by a specific synthetic agonist induces modulation of downstream signal transduction pathways which include the Wnt and the NF-kB, resulting in tumor growth inhibition (1-5).
In vivo studies have shown that A3AR agonists inhibit the development of colon, prostate and pancreatic carcinomas as well as melanoma and hepatoma. A3AR agonists were also been shown to act as anti-inflammatory agents by ameliorating the inflammatory process in different experimental autoimmune models such as rheumatoid arthritis, Crohn's disease and multiple sclerosis (6-10). It was proposed also that the A2A and A3 receptors mediate the anti-inflammatory effects of methotrexate (11).
A3 adenosine receptor (A3AR) expression levels are elevated in cancer cells as compared to normal cells (12). Thus, the A3AR expression level has been described as a means for the diagnosis of cancer (13). In addition, A3AR expression levels have also been described to be elevated in peripheral blood cells of patients with colorectal cancer (14).
Several members of the GPCR class of receptors have been reported to be modulated allosterically (15), i.e. these receptors have additional binding site(s) on a receptor that are distinct from the agonist binding site (orthosteric site, orthosterically modulated receptors), but that can modulate receptor activity.
Allosteric modulation of GPCRs has been characterized most extensively for muscarinic receptors (16), and it has been suggested that allosteric modulators may provide therapeutic advantages over orthosteric agonists. Such advantages may include greater subtype selectivity and fewer side effects (15).
The adenosine receptors are natural allosteric proteins because agonist-mediated signaling by GPCRs requires a conformational change in the receptor protein transmitted between two topographically distinct binding sites, one for the agonist and another for the G protein. Allosteric sites on GPCRs represent novel drug targets because allosteric modulators possess a number of advantages over classic orthosteric ligands, such as a ceiling level to the allosteric effect and a potential for greater GPCR subtype-selectivity.
Allosteric modulation of A1 adenosine receptors was reported (17). A number of aminobenzoylthiophenes, including PD81723, were allosteric modulators of the A1 adenosine receptor (19). These compounds were shown to be highly subtype-selective enhancers for A1 adenosine receptors (19) and were less likely to cause desensitization and down-regulation of receptors than selective A1 adenosine receptor agonists.
Some 1H-imidazo-[4,5-c]quinoline derivatives were described as selective allosteric enhancers of human A3 adenosine receptors (20). Specifically, the derivatives were shown to potentiate the potency and maximal efficacy of agonist-induced responses while decreasing the dissociation of the agonist N6-(4-amino-3-[125I]iodobenzyl)-5′-N-methylcarboxamidoadenosine from human A3 adenosine receptors.